Hikers and mountain bikers entering O'Neill Regional Park in Rancho Santa Margarita are warned of potential encounters with mountain lions, rattlesnakes and Africanized bees.
But those dangers may pale next to the presence of small, unexploded, half-century-old Navy bombs.
Over a 28-hour period beginning Wednesday morning, workers building an asphalt bike path through the park discovered 11 military bombs while grading about a foot deep near a path used by mountain bikers. The bombs were in three clusters within 100 yards of each other. One bomb in each group was live and had to be detonated by the Orange County sheriff's bomb squad.
The incidents didn't surprise county or city officials. The park had been part of the Plano Trabuco bombing range, where Navy pilots flying out of the former El Toro base from 1944 to 1956 practiced dropping ordnance. But county officials said they figured the chances of uncovering any bombs was remote.
"In over [two decades] of managing the park, we hadn't found any," said Ken R. Smith, the county's director of public works.
When workers unearthed 8-inch-long practice bombs for a third time Thursday afternoon, construction was halted on the 2.5-mile bike path project that began in late August.
On Tuesday, city, county, state and federal officials are expected to formulate a plan for clearing the wilderness area of the bombs.
County Supervisor Tom Wilson said he wants to prevent the kind of tragedy that occurred in San Diego 20 years ago, when two 8-year-olds were killed and a third youngster was injured by a live shell that exploded on a trail near Tierrasanta.
That subdivision was built on the artillery testing ground of Camp Elliott, a former Marine Corps installation.
"I'm very pleased we're out there doing the job of finding these bombs, so some visitor from the area or one of our constituents didn't come upon it and seriously injure or kill themselves," Wilson said. "We're going to make sure we proceed cautiously and try to find out what the next appropriate step is."
Mayor Gary Thompson said he hopes to delay the bike-path project until the entire park is swept clean of any explosives.
"My concern is that with the 1-foot-deep grading, they're possibly dislodging other bombs that a kid might find," Thompson said. "Let's stop the work and assess the real problem."
Unexploded munitions buried in open spaces continue to haunt Rancho Santa Margarita and hundreds of other communities in the United States.
The Defense Department recently estimated that there are 1,030 former military sites in private or public ownership, and at least 620 of them are believed to contain unexploded ordnance.
The Army Corps of Engineers, the agency still responsible for clearing ordnance from the former Plano Trabuco bombing range, is focusing on the nation's most dangerous sites first. Cleanup of the Plano Trabuco site has been priced at $2.7 million, according to a 2001 General Accounting Office report to Congress. But city officials have been told there won't be enough money in the corps' budget to start the job until 2023.
"I think these latest incidents make the case that we need help before 2023," Thompson said.
Mark Cleary, a competitive runner who said he has logged more than 5,000 miles in O'Neill park, wonders what it's going to take to get some action.
"Maybe I've been lucky," he said. "Sometimes I'll go down there twice a day. I hope it's not going to take somebody getting their foot blown off before they do something about this."